A typical filter assembly comprises a filter element and end caps secured to opposite ends of the filter element. There are various methods for attaching the end caps to the filter element, such as bonding, injection molding, and thermal welding. For example, in bonding, an adhesive or a solvent is applied to the end cap and/or the end surface of the filter element, and the filter element is pressed against the end cap until the joint is set. However, it is difficult to achieve a leak-proof seal by this method. There have been a number of proposals of forming an end cap by injection molding the end cap directly onto the end of a filter element, but such methods are slow, and the filter element must be very dense to prevent the injection pressure from driving molten plastic into the filter element and reducing the usable surface of the filter element. In thermal welding, a thermoplastic end cap is heated to liquefy one surface of the end cap and form a molten plastic. One end of a filter element is then placed against the liquefied surface of the end cap. When the plastic solidifies, the filter element is securely joined to the end cap. However, during the welding process, an undesirably large amount of molten plastic may be drawn into the filter element by capillary action, thereby degrading the properties of the end portion of the filter element.
It has also been proposed to connect end caps to filter elements by spin welding. In spin welding, two members to be joined are placed in frictional contact with one another, and then one of the members is rotated with respect to the other member to generate heat by frictional contact. The heat melts the surface of one or both of the members to form a molten material at the interface of the two members. When the rotation is stopped, the molten material solidifies to securely bond the two members to one another. Usually, one of the two members to be joined by spin welding is made of a thermoplastic resin.
Canadian Patent No. 824,484 describes one particular spin welding method for attaching an end cap to a filter element in which a cup-shaped end cap having inner and outer annular flanges is spin welded to the end of a filter element with a corrugated filter pack. However, when a cup-shaped end cap such as that described in Canadian Patent No. 824,484 is rotated with respect to a filter element, if the end cap and the filter element are not perfectly coaxial, or if there is any interference between the surfaces of the flanges and the peripheral surfaces of the filter element, the flanges of the end cap and the peripheral surfaces of the filter element may heat up before the end surfaces of the end cap or the filter element. This produces flash, i.e., an extraneous molten material, which extends over the flanges of the end cap. The flash is not only unsightly but can also damage the filter element. Furthermore, the rotating flanges of a cup-shaped end cap can damage the peripheral surfaces of a corrugated filter pack, causing the filter to leak and lose its efficiency. Therefore, for many types of filter elements, it is impractical to attach cup-shaped end caps by means of spin welding.
In addition, because conventional spin welding methods such as that described in Canadian Patent No. 824,484 use a cup-shaped end cap, the maximum dimensions of the completed filter assembly are determined by the outer diameter of the end cap, which is larger than the diameter of the filter element. When the filter assembly is installed in a housing, the housing must be at least as large in diameter as the end caps, and in most cases, this results in the housing being larger than is required by fluid flow area considerations. Furthermore, when a plurality of filter assemblies are disposed in a single housing, the number of filter assemblies that can fit into the housing is unduly limited by such unnecessarily large diameter end caps.